The present invention relates to methods of depolymerizing terephthalate polyesters, and more specifically to methods of depolymerizing post-consumer poly(ethylene terephthalate) and/or poly(butylene terephthalate) for production of terephthalate diester monomers.
Recycling of plastic materials has an important role in the 21st century in reducing environmental pollution and saving petroleum resources. PET is an engineering thermoplastic widely used in clothing fibers, packaging films, food containers and beverage bottles, that contributes several billion pounds of waste to landfills every year. The recycling rate of PET bottles in the U.S. remains 27% as of 2008, lower than aluminum cans (48%). There are mainly two conventional methods of processing post-consumer PET, mechanical recycling and chemical recycling.
Mechanical recycling is the most commonly practiced, which entails melt-processing and remolding the post-consumer PET to form products other than beverage bottles due to the deterioration of the intrinsic viscosity of the PET during the melt process. Various additives and metal catalysts contained in PET bottles promote thermal degradation of the polyester chains. The thermal stability has been improved by utilizing a solid state polymerization process that employs catalysts in the post-consumer PET to increase and/or maintain the molecular weight high enough for the fabrication process (JP 2000-219728). However, there are practical concerns such as the color tone of the product when colored bottles are used as raw materials, and control over the polymerization rate of the post-consumer PET when using different amounts and types of catalysts.
Chemical recycling yields a high quality terephthalate starting material via the chemical breakdown of PET, but has the disadvantage of high energy consumption and overall higher processing cost compared to mechanical recycling. Energy consumption can be lowered with catalysts, such as zinc acetate, aluminum isopropoxide, sodium sulfate, NaOH, KOH, and zeolite, which have been reported to be effective catalysts for the depolymerization of post-consumer PET. However, some catalysts require pressurization and long reaction times. Another drawback is that many catalysts include a non-biodegradable metal in their chemical formulas, which can become a pollution factor unless recovered and reused.
Initiatives in the chemical recycling of PET are thus ideally focused on developing an environmentally safe, economically feasible, and industrially applicable process for wide-scale application. Chemical recycling methodologies that are energy efficient and do not involve heavy metals are highly desirable even though the catalysts are usually not contained in the purified monomers.